Each venue has its own curfew, though many venues in the same area will have the same curfew, typically for logistical reasons (but sometimes for "legal" reasons as it was in this case being under the jurisdiction of the Chicago Park District). In San Francisco, because the last BART train leaves (depending which stop) around 12:30, you never really get shows that go past midnight (or you see people taking off early to catch their trains). I realize it really sucks to go to a show and have the venue get evacuated perhaps due to the CPD being overcautious, but in this type of situation with a huge crowd, I'm all for abiding by the "better safe than sorry" rule. Hopefully Phish makes up for it tonight for all the phans that made the trek!